Publications

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• Quantum enhanced metrology in the search for fundamental physical phenomena

  Konrad W. Lehnert

  SciPost Phys. Lect. Notes 40 (2022) · published 11 March 2022 | Toggle abstract · pdf

  These notes summarize lectures given at the 2019 Les Houches summer school on Quantum Information Machines. They describe and review an application of quantum metrology concepts to searches for ultralight dark matter. In particular, for ultralight dark matter that couples as a weak classical force to a laboratory harmonic oscillator, quantum squeezing benefits experiments in which the mass of the dark matter particle is unknown. This benefit is present even if the oscillatory dark matter signal is much more coherent than the harmonic oscillator that it couples to, as is the case for microwave frequency searches for dark matter axion particles.

  4 citations

• Non-Hermitian quantum quenches in holography

  Sergio Morales-Tejera, Karl Landsteiner

  SciPost Phys. 14, 030 (2023) · published 10 March 2023 | Toggle abstract · pdf

  The notion of non-Hermitian $\mathcal{PT}$ symmetric quantum theory has recently been generalized to the gauge/gravity duality. We study the evolution of such non-Hermitian holographic field theories when the couplings are varied with time with particular emphasis on the question non-unitary time vs. unitary time evolution. We show that a non-unitary time evolution in the dual quantum theory corresponds to a violation of the Null Energy Condition (NEC) in the bulk of the asymptotically AdS spacetime. We find that upon varying the non-Hermitian coupling the horizon of a bulk AdS black hole shrinks. On the other hand varying the Hermitian coupling in the presence of a constant non-Hermitian coupling still violates the NEC but results in a growing horizon. We also show that by introducing a non-Hermitian gauge field the time evolution can be made unitary, e.g. the NEC in the bulk is obeyed and an exactly equivalent purely Hermitian description can be given.

  4 citations

• Robust semi-Dirac points and unconventional topological phase transitions in doped superconducting Sr2IrO4 tunnel coupled to t2g electron systems

  Mats Horsdal, Timo Hyart

  SciPost Phys. 3, 041 (2017) · published 22 December 2017 | Toggle abstract · pdf

  Semi-Dirac fermions are known to exist at the critical points of topological phase transitions requiring fine-tuning of the parameters. We show that robust semi-Dirac points can appear in a heterostructure consisting of superconducting Sr2IrO4 and a t2g electron system (t2g-ES) without fine-tuning. They are topologically stable in the presence of the symmetries of the model, metallic t2g-ES and a single active band in Sr2IrO4. If the t2g metal is coupled to two different layers of Sr2IrO4 (effectively a multiband superconductor) in a three-layer-structure the semi-Dirac points can split into two stable Dirac points with opposite chiralities. A similar transition can be achieved if the t2g-ES supports intrinsic triplet superconductivity. By considering Sr2RuO4 as an example of a t2g-ES we predict a rich topological phase diagram as a function of various parameters.

  4 citations

• Transient Features in Charge Fractionalization, Local Equilibration and Non-equilibrium Bosonization

  Alexander Schneider, Mirco Milletari, Bernd Rosenow

  SciPost Phys. 2, 007 (2017) · published 25 February 2017 | Toggle abstract · pdf

  In quantum Hall edge states and in other one-dimensional interacting systems, charge fractionalization can occur due to the fact that an injected charge pulse decomposes into eigenmodes propagating at different velocities. If the original charge pulse has some spatial width due to injection with a given source-drain voltage, a finite time is needed until the separation between the fractionalized pulses is larger than their width. In the formalism of non-equilibrium bosonization, the above physics is reflected in the separation of initially overlapping square pulses in the effective scattering phase. When expressing the single particle Green's function as a functional determinant of counting operators containing the scattering phase, the time evolution of charge fractionalization is mathematically described by functional determinants with overlapping pulses. We develop a framework for the evaluation of such determinants, describe the system's equilibration dynamics, and compare our theoretical results with recent experimental findings.

  4 citations

• Application of Lorentzian CFT Principal Series Representation to Near Forward Scattering

  Pulkit Agarwal, Richard C. Brower, Timothy G. Raben, Chung-I Tan

  SciPost Phys. Proc. 10, 026 (2022) · published 11 August 2022 | Toggle abstract · pdf

  We present a discussion on recent progress in high energy diffraction from the perspective of AdS/CFT, through which a unified treatment for both perturbative and nonperturbative Pomeron emerges. By working with Unitary Irreducible Representation of Conformal group, a frame is provided in extending AdS/CFT to both forward and nearforward scattering. We present an analysis involving an exact solution to conformal blocks in Minkowski CFT and discuss possible applications. Phenomenological applications can range from forward scattering to DIS/DVCS/TMD at LHC energies and beyond.

  4 citations

• The QCD Equation of State in Small Systems

  W. A. Horowitz, Alexander Rothkopf

  SciPost Phys. Proc. 10, 025 (2022) · published 11 August 2022 | Toggle abstract · pdf

  We present first results on finite system size corrections to the equation of state, trace anomaly, and speed of sound for two model systems: 1) free, massless scalar theory and 2) quenched QCD with periodic boundary conditions (PBC). We further present work-in-progress results for quenched QCD with Dirichlet boundary conditions.

  4 citations

• Effect of next-nearest neighbor hopping on the single-particle excitations at finite temperature

  Harun Al Rashid, Dheeraj Kumar Singh

  SciPost Phys. 16, 107 (2024) · published 22 April 2024 | Toggle abstract · pdf

  In the half-filled one-orbital Hubbard model on a square lattice, we study the effect of next-nearest neighbor hopping on the single-particle spectral function at finite temperature using an exact-diagonalization + Monte-Carlo based approach to the simulation process. We find that the pseudogap-like dip, existing in the density of states in between the Néel temperature $T_N$ and a relatively higher temperature scale $T^*$, is accompanied with a significant asymmetry in the hole- and particle-excitation energy along the high-symmetry directions as well as along the normal-state Fermi surface. On moving from ($\pi/2, \pi/2$) toward $(\pi, 0)$ along the normal state Fermi surface, the hole-excitation energy increases, a behavior remarkably similar to what is observed in the $d$-wave state and pseudogap phase of high-$T_c$ cuprates, whereas the particle-excitation energy decreases. The quasiparticle peak height is the largest near ($\pi/2, \pi/2$) whereas it is the smallest near $(\pi, 0)$. These spectral features survive beyond $T_N$. The temperature window $T_N \lesssim T \lesssim T^*$ shrinks with an increase in the next-nearest neighbor hopping, which indicates that the next-nearest neighbor hopping may not be supportive to the pseudogap-like features.

  4 citations

• Tensor network variational optimizations for real-time dynamics: Application to the time-evolution of spin liquids

  Ravi Teja Ponnaganti, Matthieu Mambrini, Didier Poilblanc

  SciPost Phys. 15, 158 (2023) · published 12 October 2023 | Toggle abstract · pdf

  Within the Projected Entangled Pair State (PEPS) tensor network formalism, a simple update (SU) method has been used to investigate the time evolution of a two-dimensional U(1) critical spin-1/2 spin liquid under Hamiltonian quench [Phys. Rev. B 106, 195132 (2022)]. Here we introduce two different variational frameworks to describe the time dynamics of SU(2)-symmetric translationally-invariant PEPS, aiming to improve the accuracy. In one approach, after using a Trotter-Suzuki decomposition of the time evolution operator in term of two-site elementary gates, one considers a single bond embedded in an environment approximated by a Corner Transfer Matrix Renormalization Group (CTMRG). A variational update of the two tensors on the bond is performed under the application of the elementary gate and then, after symmetrization of the site tensors, the environment is updated. In the second approach, a cluster optimization is performed on a finite (periodic) cluster, maximizing the overlap of the exact time-evolved state with a symmetric finite-size PEPS ansatz. Observables are then computed on the infinite lattice contracting the infinite-PEPS (iPEPS) by CTMRG. We show that the variational schemes outperform the SU method and remain accurate over a significant time interval before hitting the entanglement barrier. Studying the spectrum of the transfer matrix, we find that the asymptotic correlations are very well preserved under time evolution, including the critical nature of the singlet correlations, as expected from the Lieb-Robinson (LR) bound theorem. Consistently, the system (asymptotic) boundary is found to be described by the same Conformal Field Theory of central charge $c = 1$ during time evolution. We also compute the time-evolution of the short distance spin-spin correlations and estimate the LR velocity.

  4 citations

• Dark matter from the centre of SU(N)

  Michele Frigerio, Nicolas Grimbaum-Yamamoto, Thomas Hambye

  SciPost Phys. 15, 177 (2023) · published 25 October 2023 | Toggle abstract · pdf

  A dark sector with non-abelian gauge symmetry provides a sound framework to justify stable dark matter (DM) candidates. We consider scalar fields charged under a $SU(N)$ gauge group, and show that the centre of $SU(N)$, the discrete subgroup $Z_N$ also known as $N$-ality, can ensure the stability of scalar DM particles. We analyse in some details two minimal DM models of this class, based on $SU(2)$ and $SU(3)$, respectively. These models have non-trivial patterns of spontaneous symmetry breaking, leading to distinctive phenomenological implications. For the $SU(2)$ model these include a specific interplay of two DM states, with the same interactions but different masses, and several complementary DM annihilation regimes, either within the dark sector or through the Higgs portal. The $SU(3)$ model predicts dark radiation made of a pair of dark photons with a unique gauge coupling, as well as regimes where DM semi-annihilations become dominant and testable.

  4 citations

• On quantum melting of superfluid vortex crystals: From Lifshitz scalar to dual gravity

  Dung Xuan Nguyen, Sergej Moroz

  SciPost Phys. 17, 164 (2024) · published 11 December 2024 | Toggle abstract · pdf

  Despite a long history of studies of vortex crystals in rotating superfluids, their melting due to quantum fluctuations is poorly understood. Here we develop a fracton-elasticity duality to investigate a two-dimensional vortex lattice within the fast rotation regime, where the Lifshitz model of the collective Tkachenko mode serves as the leading-order low-energy effective theory. We incorporate topological defects and discuss several quantum melting scenarios triggered by their proliferation. Furthermore, we lay the groundwork for a dual non-linear emergent gravity description of the superfluid vortex crystals.

  4 citations